This invention relates generally to implantable electrical leads, and in particular to electrical stimulation leads.
In the early days of pacing, a cardiac pacing lead was viewed simply as a wire connecting the pacemaker to the heart. However, those skilled in the art have come to appreciate that a cardiac pacing lead as implanted is part of a complicated electrical, mechanical and chemical system.
In an effort to improve performance, manufacturers of pacing leads have selected specific commercially available alloys which have particularly advantageous mechanical and electrical properties when used in pacing leads. These include stainless steels, Elgiloy.RTM. alloy, MP35N alloy, and DBS/MP. DBS is a drawn-brazed-strand, having a silver core surrounded by strands of stainless steel or of MP35N alloy. All of these conductors, when coiled, display appropriate mechanical and electrical characteristics for use in electrical stimulation leads.
Although most early pacing leads were fabricated using silicone rubber to insulate the conductors, manufacturers have become aware of the superior mechanical properties of commercially available polyether urethanes. These include Pellethane 80A and Pellethane 55D polyurethanes manufactured by Dow Chemical Company. These polyurethanes are less thrombogenic than silicone rubber and higher in tensile strength. In addition, they slide easily against one another when moistened with body fluids. This property facilitates the use of two leads in a single vein, which was difficult with the older silicone rubber bodied leads. Unfortunately, recent experience has suggested that cobalt, chromium and molybdenum, commonly used in lead conductors, may accelerate oxidative degradation of polyurethanes used in pacing leads. MP35N, Elgiloy and DBS/MP all include cobalt, molybdenum and chromium as significant constituents. To a lesser degree, it appears that stainless steels may also accelerate polyurethane degradation.
An additional set of improvements in implantable electrical leads has been the trend toward fabrication of multiconductor coils, rather than separate, mutually insulated coils. Early leads, such as those disclosed in U.S. Pat. No. 3,348,548 and U.S. Pat. No. 3,788,329 show separate conductor coils in a side by side or coaxial configuration, insulated from one another by sheaths covering the entirety of the coils. More recently, multipolar coiled conductors having individually insulated coil wires have been pursued, as disclosed in Canadian Patent No. 1,146,228, for a Multipolar Pacing Conductor, issued May 10, 1983 to Upton. This patent discloses a single, multiconductor DBS coil having individually insulated wires, appropriate for use in conjunction with a polyurethane outer insulation and is incorporated herein by reference in its entirety.